For example, an apparatus for forming a ZnO epitaxial film for a white light emitting diode (LED) or another light emitting element forms a ZnO film on a substrate by causing source gases react together, which may be an O source gas and a Zn source gas.
The substrate is mounted in the apparatus, in which the source gases are blown through the respective supply ports onto the substrate. The source gases flow through separate passages to the respective supply ports so that the gases cannot react until they reach the ports. However, if the passages are sealed insufficiently, the source gases react together before they reach the supply ports. This results in crystal film formation (of ZnO) on the inside of the passages etc., worsens the gas crystallizability on the substrate, lowers the film growth speed and causes particle deposition.
The source gases flowing from the supply ports toward the substrate react together before they reach the substrate. This results in very small portions of the source gases being consumed for the film formation on the substrate, so that the formation requires a great quantity of source gas and a long time.
In particular, if one of the source gases is an O source gas (O2, H2O, N2O or the like), it is apt to react in its gaseous phase with the other, so that high-quality crystal film formation cannot be performed on the substrate. This problem is difficult to solve because the O source gas is highly reactive even if the reaction chamber of the apparatus is in a high-vacuum state. The problem is more difficult to solve if the reaction chamber is in a low-vacuum state, which makes the O source gas more reactive.
The supply ports are positioned where they are subject to the radiation heat from the substrate, so that the source gases start to react immediately after they leave the ports. This worsens the gas crystallizability on the substrate.
In the reaction chamber, while the source gases are flowing toward the substrate, they are induced by the exhaust suction. Consequently, part of the source gases forms a reaction product near the substrate before they form a film on the substrate. This likewise worsens the gas crystallizability on the substrate, lowers the film growth speed and causes particle deposition.
Patent Document 1: Japanese Patent No. 3,198,956
Patent Document 2: Japanese Unexamined Patent Publication No. H1-101623
In order to perform high-quality crystal film formation on the substrate, it is necessary to completely seal the passages for the source gases in the apparatus so that the gases cannot react while flowing through the passages. It is also necessary to prevent the source gases from reacting too early while flowing from the supply ports to the substrate. It is important to position the supply ports so as to minimize the influence of the radiation heat from the substrate. It is further necessary to prevent the behavior of the source gases in the reaction chamber from being disturbed by the exhaust suction.